A typical squall line taking place in Beijing area is analyzed with the Sband Doppler weather radar data and intensive surface AWS observations in Beijing. The results indicate that: (1) meteorological elements show abrupt changes during the influence of the squall line, such as mutation in the wind direction, surge in wind speed, drop of temperature and steep rising of pressure; (2) there is a good relationship among surface vorticity, precipitation and thunderstorm winds. The position of precipitation and the variation of the distance between precipitation and positive vorticity can be used to indicate the future trend of convective cells; (3) surface water vapor flux shows some small scale characteristics, such as suface outflow of thunderstorm, terrain convergence and lowlevel water vapor transport.The combination of positive water vapor flux from thunderstorm’s outflow and terrain convergence has strengthened the convergence and lift in the front of the mountain, this is the main reason for the quickly development of thunderstorm in the front of mountain and subsequently local heavy rain and thunderstorm winds. The surface water vapor flux can be used as an indicator of the future trend for the thunderstorms.